This invention relates generally to control systems for controlling the temperature of multiple zoned space, such as a building, and more particularly to a variable air volume digital control system having a temperature system manager device which determines an air flow setpoint in a temperature zone as a percentage of the total air volume of a terminal in the temperature zone and as a function of the deviation of a local temperature sensor temperature from a setpoint schedule in the temperature system manager device, whereby the flow adjustments made by a local terminal control unit are at a higher rate than the rate of receipt by the terminal control unit of a flow target set by the temperature system manager device.
The utilization of variable air volume (VAV) air distribution systems to supply conditioned air from a central source thereof to offices, school rooms, and other similar spaces or areas in multi-room buildings has become increasingly more prevalent. Such VAV systems generally furnish varying volumes of air, at constant temperatures, into a space in accordance with the space or zone demands. The flow of conditioned air from outlets or terminals is generally regulated by operation of suitable damper means controlled by a thermostat sensing the temperature of the space being conditioned. Thus, as the temperature of the space deviates to a greater degree from a predetermined setpoint, the damper opens more as a direct result of the deviation and a greater quantity of conditioned air is discharged into the space. Conversely, when the temperature in the space being conditioned approaches the setpoint, the system decreases the air volume to the space depending upon the deviation of the space temperature from the setpoint. In U.S. Pat. No. 4,756,474 assigned to the same assignee as the present invention, there is described a pneumatic controller for a duct pressure powered air terminal unit having a volume controller which receives two pressure signals, whereby the controller bleeds one pressure signal so as to control the inflation of a bellows or bladder to thereby modulate the terminal unit to maintain a desired volume air flow through the unit, and bleeds the second pressure signal so as to maintain at least a minimum flow through the unit. The above-identified controller is an improvement over U.S. Pat. No. 4,120,453 which describes a three-way valve controller having two pressure regulators and a bleed type thermostat which provide four input signals to the three-way valve thereby providing a single pressure signal to the inflatable bellows.
In these prior systems, a target flow of air discharged to the space was based on a proportional-only term described as the difference between Temperature Setpoint and Actual Temperature multiplied by an empirically-derived constant of proportionality (k). This proportional-only term is used to modify, either up or down, the flow of air into the space as required, in order to maintain Temperature setpoint in the space. The empirically derived constant of proportionality (k) is fixed in value and thus supports only the fixed load characteristics as were exercised in empirically deriving the constant of proportionality (k) in the first place. These fixed load characteristics are typically the size of the heating or cooling load relative to heating or cooling capacity of the space, and the rate of change of the gross heating or cooling load. However, any variation in these factors in the actual building or space will result in a compromise in the control's ability to maintain the temperature setpoint.
Also in these prior systems the actual target flow is calculated by a centralized device, such as personal computer, electrically connected and communicating to several control devices for the damper means that regulate the flow of air into their corresponding spaces. A partial or catastrophic failure of this central computing device, or a partial or catastrophic failure of the electrical connection to the damper controller devices will result in a significant compromise in the temperature control in the space, since target flow updates are no longer possible. Thus single-point failures may result in complete loss of temperature control in a building.
Thus there is a clear need for a method and apparatus to determine, on a temperature zone basis, and in regard to actual and dynamic load conditions present, a flow (CFM) setpoint based on proportional, integral and derivative error terms and as a percent of air volume delivered by a terminal to the temperature zone on deviation of zone temperature from a scheduled setpoint signal, and then generate a higher rate of flow signals to be supplied to the air terminal to make multiple, and thus pressure independent, adjustments to the flow of air to the temperature zone for each flow setpoint signal generated.
Furthermore, a provision must be made for the terminal control unit or damper controller to recognize the absence of communication from the centralized computing device that typically calculates the flow setpoint and determine at that point in time that a failure of the system has occurred, at which time the local air terminal or damper controller will utilize a default temperature "comfort setpoint" and locally provide a proportional adjustment to air flow to the space based on error from temperature setpoint and actual temperature, and thus maintain reasonable comfort in the space until communication with the centralized computing device is re-established.